He’s the R. White’s Lemonade Secret Lemonade Drinker.
He’s also deeply sinister and disturbing… but maybe that’s just me.
This advert first aired in 1973. The song the guy in the pyjamas is miming to was written and sung by Ross MacManus, with his son Declan paying drums and providing backing vocals (“R. White’s… R. White’s…”). Nowadays, Declan is better known as Elvis Costello.
There were a few followup adverts which changed the ending of the original. Here are a couple of them: